Encryption in wireless services has become important in order to prevent cellular phone fraud, to enhance electronic commerce and to support personal privacy. Standards for mobile telephony have been established to include the requirement of voice ciphering for voice privacy as well as signaling message and data encryption, for example in CDMA (IS-95), GSM, (ETSI GSM 03.20 and GSM 03.21) and TDMA standard IS-136(2).
Various methods have been proposed to achieve the requirement of these standards. However, the various key and mask generation proposals for achieving the voice ciphering and message/data encryption are different from each other. All, so far, however utilize applying a mask bit stream to the information bit stream via an exclusive-OR (XOR) operation.
The standard IS-136 REV A includes a figure as shown in FIG. 1. A speech encoder 1 outputs 77 class-1 and 82 class-2 bits. The 12 most perceptually significant bits of the class-1 bits are applied to a 7 bit cyclic redundancy count (CRC) computation process 3 for determination of a value to be used in the receiver for error detection. The 77 class-1 bits and the 7 CRC bits, as well as 5 tail bits are applied to a rate 1/2 convolutional coder 5 for channel encoding, producing 178 coded class-1 bits. Those coded class-1 bits and the 82 class-2 bits are applied to a voice cipher circuit 7, which produces a 260 bit bit-stream. After passing through a 2-slot interweaver 9, the signal is applied to a modulator for transmission (not shown).
It should be noted that the voice ciphering is performed after rate 1/2 convolutional coding of the speech signal, and before modulation. Encryption is performed in the voice cipher circuit 7 by applying a mask to the voice bit stream via an XOR operation, bit by bit. By the term "circuit" herein is meant either or both of hardware and process, which may include software.
After transmission of the encrypted signal via e.g. a wireless medium, it is received by a receiver. In the receiver, a system which processes the signal in a manner opposite to the system shown in FIG. 1 is used. It should be noted that the received signal is demodulated, deciphered, and then channel decoded before being sent to a speech decoder. The information sequence is represented as bits (referred to below as bit-wise operation) before being deciphered because the XOR operation and the mask bit stream is required to be used. Thus, bit-wise operation is used before modulation in the transmitter and right after demodulation in the receiver. This is a major roadblock preventing soft-decision decoding from being used for this application, for the following reasons.
FIG. 2 illustrates the encryption and decryption technique in the prior art system in more detail. A data bit stream is received by a channel encoder 11, and the stream of encoded data bits is applied to an XOR circuit 13 with a mask bit stream. The resulting encrypted data bit stream is applied to a modulator 15 (assumed herein to include a transmitter) to a wireless medium 17.
The signal is received and demodulated in a demodulator 19 of a receiver, which applies the encrypted bit stream to a decryption circuit 21, typically comprised of an XOR circuit, with a corresponding mask bit stream as was used in the encryption circuit. The resulting decrypted signal is applied to a hard decision decoder 23, from which a decoded bit stream is provided as an output signal.
In general, channel decoding can be performed in either of two ways, namely hard decision decoding and soft decision decoding. Usually analog samples output from the demodulator can be quantized and then decoding is performed digitally. In the extreme case in which each sample corresponding to a single bit of a code word is quantized to two levels, i.e. 0 or 1, the demodulator is said to make a hard decision and the channel decoder that works with this kind of input is said to perform hard decision decoding.
On the other hand, if the quantization is more than two levels, the resulting quantized samples are called soft symbols, or simply, symbols. The channel decoder that makes use of the information as soft symbols is said to perform soft decision decoding.
Hard decision decoding has the advantage of less computational complexity due to the bit-wise operation. However, for the same reason some useful information is lost during quantization and therefore it does not perform very well under certain circumstances, for example, in a noisy channel. However, noisy channels are common in real wireless communication systems.
Soft decision decoding (SDD) offers significantly better performance than hard decision decoding. For example, it has been reported that to achieve the same error probability, at least 2 dB more signal power must be generated at the transmitter when the demodulator uses a hard decision output (assuming the channel is an Additive White Gaussian Noise (AWGN) channel). Put another way, there is at least a 2 dB improvement for soft decision decoding in an AWGN channel. This improvement implies an increment in the capacity of a wireless cellular system, which is one of the most important issues in the wireless industry.
It is therefore desirable to provide SDD in the receiver. This requires the input to the soft decision decoder to be symbols instead of bits. The demodulator must therefore make a soft decision to output symbols. As a result, the input and output of an encryption process must be in symbol format. However, all of the current encryption schemes are based on bit-wise XOR masking operations. This makes SDD and XOR-based encryption very difficult, if not impossible, and apparently incompatible.